1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer and the like. More particularly, it relates to an image forming apparatus wherein a transfer and separation condition can be switched in response to an output of a humidity sensor.
2. Related Background Art
Conventionally, in image forming apparatuses such as copying machines, a toner image formed on a photosensitive drum is generally transferred onto a transfer sheet by a transfer and separation device, and thereafter, the transfer sheet is separated from drum. In this case, a transfer and separation condition for the toner image (for example, a value of current applied to a transfer and separation means) is properly controlled in accordance with the humidity of a place where the image forming apparatus is installed. This is because the quality of the transfer and separation operation for the toner image depends upon the humidity. The humidity is measured by a humidity sensor incorporated into the image forming apparatus.
The humidity sensor is generally arranged on a substrate of a DC controller and the like. However, according to the conventional technique wherein such humidity sensor is arranged on the substrate, a temperature around the substrate is gradually increased by the heat generated by elements constituting the substrate as the time is elapsed after a main switch is turned ON. As a result, even when the external environment (referred to as "installation environment" hereinafter) in which the image forming apparatus is installed is constant, an output level of the humidity sensor is gradually transferred toward a low humidity side. That is to say, regardless of the constant absolute humidity, since the temperature is increased, the absolute humidity detected by the humidity sensor is gradually decreased. Thus, in a mode for controlling the transfer and separation condition on the basis of the humidity successively measured, although the transfer and separation condition suited to the installation environment can be obtained immediately after the main switch is turned ON, since the transfer and separation condition is lowered to a lower humidity side lower than the humidity of the installation environment, it is feared that a solid image (image having a wide area) becomes thin or obscure, thus causing the poor transfer.
On the other hand, in order to eliminate this disadvantage, it is considered that the humidity is measured only when the main switch is turned ON, the transfer and separation condition is determined on the basis of the measured humidity and the value of the determined condition is fixed. In this method, even when the humidity level around the humidity sensor is lowered below the humidity of the installation environment by the increase in the temperature after the main switch is turned ON, the transfer and separation condition matching to the installation environment can be maintained. However, after the main switch is turned OFF, when the main switch is turned ON again before the humidity level around the humidity sensor is returned to the humidity of the installation environment, similar to the above, since the temperature in the image forming apparatus is in the elevated temperature condition, the transfer and separation condition having the humidity lower than the humidity of the installation environment is selected, with the result that it is feared that the transfer void can easily occur.
Incidentally, it is considered that only the humidity sensor is installed at a location which is not influenced upon the temperature increase. However, in this case, it is necessary to add any member for installing the humidity sensor, thus making the installing operation troublesome and making the apparatus expensive.